Interesting post and exchanges by Ckret etal 2008, on oscillations vs. bump:
Ckret
• 522 posts #4934
November 2, 2008
<Quote Snowmman>
Didn't open the jpg's to look at the report you were referencing, I was speaking to the jump (equipement used and how it would have performed in a higher low opening) which was calculated by a member of the Boeing jump club, of which all were investigated.
The problem with the report is they used the time of the oscillations for the jump, which i think is wrong. i think when you put the whole jump together the oscillations were Cooper slowly moving down the stairs. The bump didn't occur until a few minutes later; when they could see Portland just from the north.
So all of the calculations are correct, just have to move everything south a few minutes.
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Reply>
In order for something to "oscillate" you have to have some force(s) being applied where a sympathetic frequency is set up. The logical force is the wind, or cross winds. The logical event is the stairs extending into the air stream.
But this happens at about 20:12 precisely when they make a left hand turn in the approach to BTGVOR, which changes the angle of the aircraft relative to the wind(s). So, it is not required that the stairs are being extended further, only that the flight angle of the (plane with stairs) changes with respect to the airstream.
Also there is nothing in the transcript which says the oscillations increased or decreased after they began or if they went away? prior to the bump?
The test which confirmed the bump also confirmed oscillations prior to the bump? And what exactly was "oscillating"
Just an observation -
Georger[/reply]
You are applying too much science, over over analyzing, sometimes the answers are simple. From putting everything together, the crew was referencing the cabin pressure gauge when the statement of “oscillation” was made.
Not that they were feeling an oscillation in the aircraft. Remember; in another log created at the same time as the one reporting oscillations the word used was “
fluctuations.”
Because the crew always referenced the bump as a pressure change it would make since that prior to the large “Pressure Event,” there were small events leading up.
Now go back and you’re Cooper getting ready to jump. You are gingerly walking down the air stairs because you have never done this before. With each step you take caution, take a step and see what happens, take a step and see what happens. As you are doing this, the cabin pressure gauge in the cockpit starts to fluctuate.
The engineer notices this and reports the anomaly to ops, these fluctuations continue for a few minutes as you figure things out. Once you’re set you jump and the stairs come back to the body of the plane, causing the pressure event known as “the bump.” It is noticeable but not dramatic, the needle instantly spikes, they notice the change in their ears, things calm quickly. Because the small fluctuations continue (the stairs are still open to about 15 degrees) nothing is reported they just assume this is a continuation of what they have already reported. But they do take note, “wonder if he just jumped?” As they wonder this they notice they are just north of the Portland suburbs.
I think the individual typing the teletype was being fed info from the guy keeping the hand written log. When the guy keeping the handwritten log said, “they are reporting some type of fluctuations in the cabin pressure; they said the gauge is bouncing.” The teletype operator then typed “oscillations” his words not the crews.
Amazing how small, even seeming harmless interpretations can cause large fluctuations or oscillations in history.
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[reply] Georger
This whole scenario is mixing and confusing multiple effects and causes. Vibration of stairs. Trim of aircraft? These are mechanical effects. Then there are acoustic effects being seen on the cabin pressure gauge related to the opening and closing of the hole in the back of the plane. Both effects happening simultaneously but they are separate physical events each with a different cause. The large pressure spike called a 'bump' is an acoustic event due to the sudden closing of the hole by the stair door slamming and closing the rear hole for an instant of time. The crew probably didn't expect that because they had never experienced that happening in a 727 before. The guys dropping cargo in Vietnam may have experienced that - you should check with them? Immediately after the large acoustic pressure spike (door closing) they say the vibrations stopped. That's key! That's the time and place Cooper jumped imo. But there are two separate physical events happening in this oscillations vs bump scenario.